[mlir][vector] Add unroll pattern for vector.gather

This pattern is useful for SPIR-V to unroll to a supported vector size
before later lowerings. The unrolling pattern is closer to an
elementwise op than the transfer ops because the index values from which
to extract elements are captured by the index vector and thus there is
no need to update the base offsets when unrolling gather.

Differential Revision: https://reviews.llvm.org/D149066